Controllable pitch marine propellers provide numerous advantages over fixed pitch propellers, especially in vessels that operate at various speeds with varying loads. A well-known form of controllable pitch propeller is the force rod type, in which a blade pitch control mechanism in the propeller hub is operated by a force rod that extends through the shaft and is moved forward and aftward by an inboard hydraulic servo or other suitable means. The pitch-control mechanism in the hub comprises a cross-head affixed to the aft end of the force rod and coupled to each of the blades by crank pins and slideways. In some designs the slideways are on the cross-head and the pins on the blade mounts. In other designs the pins are on the cross-head and work in corresponding slideways on the blade mounts. The blades are, of course, mounted on the hub for rotation about pivot axes disposed radially of the propeller shaft axis, and the crank pins and slideways are offset generally circumferentially from the pivot axes of the blades.
In the majority of present designs of force rod type controllable pitch propellers, the cross-head is generally a block of metal attached to the end of the force rod and containing slideways for each crank pin sliding shoe. It is located within the propeller hub such that at a neutral (zero pitch) position it lies radially inwardly of the blade-mounting trunions. Upon movements for ahead and astern pitch control the cross-head moves forward and aftward from the neutral position. With this configuration, the open region of the hub swept by the cross-head in its pitch-controlling motions has to be aftward of the aft end of the shaft. Accordingly, the shafting and aftmost shaft bearing have to be designed to carry a substantial overhung moment due to the load of the propeller, which is located almost entirely aftward of the aft end of the shaft.
There are two principal ways of attaching a propeller to a tailshaft. One is to provide an externally tapered end on the shaft and a matching internally tapered mount spigot in the propeller hub. The hub is driven tightly onto the taper and fastened by a large nut. The other involves forging a flange onto the shaft end and bolting the hub to the flange. The flange mount is significantly more expensive than the taper mount and is, therefore, rarely used in smaller propellers, say those less than 12 feet in diameter and 3000 H.P.
It is known to weld coupling flanges onto sections of a propeller shaft that are located inboard of the vessel hull. Welded flanges inboard of the hull are loaded primarily in torsion and only lightly loaded in flexure, because the shaft itself is relatively light in weight and is supported by bearings. Accordingly, the strength requirements for the weldments between the shaft and coupling flange are not great. Also, the inboard weldments are not immersed in water, and corrosion is not a problem.
A flange-mounted propeller presents a very different situation. First, the flange has to carry the very high, cyclical flexural loads resulting from the overhang moment of the propeller, as well as high torsional loads. Second, the flange is immersed in water. Because of the high loads and the exposure to corrosion, welded propeller mount flanges have not been used heretofore.